A magnetic bearing, which includes a rotor and a stator concentrically located with respect to each other, typically controls the radial or axial distance between the rotating rotor and the stationary stator. More specifically, adjustable electro-magnetic forces generated by current flowing through coils wrapped around the stator poles, as controlled by a control circuit adjusts distances between the stator and rotor. U.S. Pat. No. 4,387,935 and 4,082,376 describe details of the magnetic bearing.
Although superior to mechanical bearings in terms of rotational losses, magnetic bearings exhibit rotational losses caused mainly by eddy-current losses generated when non-uniform flux distributions exist along the rotor surface. A heteropolar magnetic bearing requires reversal of the magnetic bias-flux direction as seen by the rotor at each stator pole location resulting in a greatly varying flux distribution around the rotor. Laminated soft-magnetic material is commonly used for the rotor construction to reduce eddy-current losses.
Homopolar magnetic bearings are known to minimize these losses by utilizing two stators: one feeds magnetic bias flux into its rotor and the other feeds magnetic bias flux out of its rotor making the flux distributions around the rotors much more uniform than for the heteropolar case. As the rotors rotate, flux reversal, which is inherent in heteropolar magnetic bearings, will then not occur. However, flux levels as seen by the rotors will modulate with rotation because fluxes will drop off at rotor locations between the stator poles. The drop off of flux levels as seen by the rotors will maintain high eddy-current and rotational losses, but these losses will be less than those of heteropolar magnetic bearings.
The pole-to-pole gap of a magnetic bearing is generally orders of magnitude larger than its pole-to-rotor gap. There are two reasons for having a large pole-to-pole spacing. First, in order to develop force, it is necessary for flux to flow from pole to rotor (pole-to-rotor) and from rotor to pole (rotor-to-pole) to an opposing pole. A parallel flux path exists pole-to-pole which will not generate force and waste generated pole flux. Maintaining a relatively large pole-to-pole gap will cause this parallel flux path to have a high reluctance and be insignificant. Second, it is more convenient for assembly to install coil windings into a relatively large pole-to-pole gap.
If the two reasons for having a large pole-to-pole gap can be tolerated or dealt with in some manners, it can be possible to reduce the pole-to-pole gap to create a more uniform rotor flux distribution. This can result in a reduction of eddy-current losses and rotational losses.
Thus, it is an object of this invention to provide a multi-pole homopolar magnetic bearing with uniform rotor flux distribution.
It is a further object of this invention to provide a multi-pole homopolar magnetic bearing that reduces eddy current and rotational losses therein.
It is yet a further object of this invention to provide a multi-pole homopolar magnetic bearing with sectored-pole-pieces.
It is still another object of this invention to provide a multi-pole homopolar magnetic bearing with reduced pole-to-pole gaps in accordance with flux allocation ratio.